Re: ZONE_DMA

[Christoph Lameter <clameter@sgi.com>]

On Fri, 22 Sep 2006, Jesse Barnes wrote:

> Right, being able to allocate from specific ranges would obviate the need 
> for GFP_DMA and the various zones.  It would come with a cost though since 
> the VM would have to become aware of pressure at various ranges rather 
> than just on zones like we have now.  I think that's where things get 
> tricky.

Here is a hyperthetical use scenario (no code yet to do this in the page 
allocator, fake patch for discussion only):

Index: linux-2.6.18-rc7-mm1/arch/i386/kernel/pci-dma.c
===================================================================
--- linux-2.6.18-rc7-mm1.orig/arch/i386/kernel/pci-dma.c	2006-09-12 20:41:36.000000000 -0500
+++ linux-2.6.18-rc7-mm1/arch/i386/kernel/pci-dma.c	2006-09-22 13:59:29.017611573 -0500
@@ -26,6 +26,8 @@ void *dma_alloc_coherent(struct device *
 			   dma_addr_t *dma_handle, gfp_t gfp)
 {
 	void *ret;
+	unsigned long low = 0L;
+	unsigned long high = 0xffffffff;
 	struct dma_coherent_mem *mem = dev ? dev->dma_mem : NULL;
 	int order = get_order(size);
 	/* ignore region specifiers */
@@ -44,10 +46,14 @@ void *dma_alloc_coherent(struct device *
 			return NULL;
 	}
 
-	if (dev == NULL || (dev->coherent_dma_mask < 0xffffffff))
-		gfp |= GFP_DMA;
+	if (dev == NULL)
+		/* Apply safe ISA LIMITS */
+		high = 16*1024*1024L;
+	else
+	if (dev->coherent_dma_mask < 0xffffffff)
+		high = dev->coherent_dma_mask;
 
-	ret = (void *)__get_free_pages(gfp, order);
+	ret = page_address(alloc_pages_range(low, high, gfp, order));
 
 	if (ret != NULL) {
 		memset(ret, 0, size);
Index: linux-2.6.18-rc7-mm1/include/linux/gfp.h
===================================================================
--- linux-2.6.18-rc7-mm1.orig/include/linux/gfp.h	2006-09-19 09:26:58.000000000 -0500
+++ linux-2.6.18-rc7-mm1/include/linux/gfp.h	2006-09-22 14:02:34.298613635 -0500
@@ -136,6 +136,9 @@ static inline struct page *alloc_pages_n
 		NODE_DATA(nid)->node_zonelists + gfp_zone(gfp_mask));
 }
 
+extern struct page *alloc_pages_range(unsigned long low, unsigned long high,
+			gfp_t gfp_mask, unsigned int order);
+
 #ifdef CONFIG_NUMA
 extern struct page *alloc_pages_current(gfp_t gfp_mask, unsigned order);
 
-
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